What Do the Gas Octane Ratings Actually Mean?

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Refueling a car (Photo: @askoldsb via Twenty20)
If you’re a driver, the numbers 87, 89, and 92 should look pretty familiar to you at this point. You see them every time you stop for gas, and they represent the different octane ratings that are available in the U.S. But while you may know them by heart, you might not know the meaning behind them.
Here’s what those ratings actually mean and which one you should be using for your car. The next time you go pump some gas, you’ll know exactly what you are buying. 

What Does the Octane Rating Mean

The octane rating is a measure of how well the fuel can resist “knocking” during combustion. The higher the rating, the better the resistance. Vehicle engines are designed to work under a controlled combustion method. A flame starts a spark that then burns through all of the fuel in the engine cylinder. However, the high temperatures or pressure from this combustion can cause unburned fuel to ignite, and this secondary and uncontrolled combustion is what causes the “knocking.” As you might’ve guessed, higher octane-rated gas is better at resisting these combustions. 

Which Fuel Octane Number Should You Use?

As a rule of thumb, you should be using the fuel recommended by your manufacturer. You can find this information in your owner’s manual. Nowadays, most vehicles are set to run on 87-octane fuel; however, this can vary depending on the model and engine, so make sure to reference your manual. 

What Happens if You Use a Different Rating Than the One Indicated by the Manufacturer? 

That will depend on whether you go higher or lower. 
Since lower-octane fuel is cheaper to produce and, therefore, less expensive to purchase, you might be tempted to go with the lowest option possible. 
As noted, however, lower-octane fuel is more prone to uncontrolled combustion and therefore knocking. Modern engines are designed to be able to control this problem in a way; they can delay the primary combustion once they detect an uncontrolled combustion has happened. The result? No knocking.
However, this combustion regulation can cause the engine to underperform when it comes to power and fuel efficiency. And while going this route once in a while will most likely not have significant repercussions, if done consistently over time it can cause more severe and permanent damage to the engine. 
Perhaps the price is not your primary concern, but the health of your engine is. You want to take good care of your car, which might make you lean towards the high-octane fuel. 
There are indeed some benefits to using a high-octane fuel, the main one being better gas mileage. You might also benefit from lower CO2 emissions when performing heavy-duty tasks such as carrying heavy loads
It’s worth noting, though, that there’s almost no benefit to using a higher octane rating under normal, everyday driving conditions. And the savings you’d get from the improved gas mileage can be often overturned by the high price related to a higher-octane gas. Therefore, the overall benefits might not justify the higher expense.

The Bottom Line on Gas Octane Ratings

  • The octane level assesses the fuel’s ability to resist premature combustion and, therefore, “knocking.”
  • You should be using the fuel indicated by your car’s manufacturer, as stated in your owner’s manual. 
  • Going for a lower octane rating can cause damage to your engine in the long run. 
  • Going for a higher octane rating can improve gas mileage but will increase costs by a higher margin.